1. Technical Field
The present invention relates to an apparatus for driving a display device such as a liquid crystal display.
2. Description of the Related Art
A liquid crystal display (LCD) includes two panels provided with field-generating electrodes, such as pixel and common electrodes, and a liquid crystal layer interposed therebetween. An LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the liquid crystal layer which aligns the liquid crystal molecules of the liquid crystal layer to control the polarization of incident light, thereby displaying images. An LCD realizes gray scale by adjusting the voltages between the pixel electrodes and the common electrode. Generally, the gray scale ranges from the first gray representing the darkest state to the sixty-fourth gray representing the brightest state.
A vertically-aligned (VA) mode LCD, in which longitudinal axes of liquid crystal molecules are aligned perpendicular to the two panels in the absence of an electric field, can produce a relatively high contrast ratio and a relatively wide viewing angle. The contrast ratio can be defined as the luminance of the sixty-fourth gray divided by that of the first gray.
When an electric field is applied, the liquid crystal molecules are tilted with respect to an axis normal to the panels at angles dependent on the strength of the electric field, so that polarization of light passing through the liquid crystal layer can be changed. The larger the tilt angle of the liquid crystal molecules, the larger the amount of change in polarization. The wide viewing angle of the VA mode LCD may be realized by openings (cutouts) or protrusions on the field-generating electrodes. As the tilt angle of the liquid crystal molecules is determined by the openings or the protrusions, the tilt angle of the liquid crystal molecules can be increased, thereby widening the viewing angle.
However, the contrast ratio of a VA mode LCD may be poor when viewed from its lateral sides, as compared to when viewed from the front of the display. For example, in the case of a patterned vertically aligned (PVA) mode LCD having the opening in the field-generating electrodes, the luminance is enhanced toward the display's lateral sides. In the worse case, the luminance difference between high grays vanishes such that the image can not be perceived.
In one method to improve contrast ratio, a pixel is divided into two sub-pixels, and the two sub-pixels are capacitance-combined with each other. Voltage is directly applied to one sub-pixel, and a voltage drop is caused at the other sub-pixel due to the capacitance combination so that the two sub-pixels differ in voltage from each other, thereby differentiating the light transmittance thereof.
However, it is difficult to precisely control the light transmittance of the two sub-pixels. For example, as the light transmittance is differentiated for respective colors, the proper voltage distribution cannot be separately made for the respective colors. Furthermore, the aperture ratio may be deteriorated due to the addition of a conductor for the capacitance combination and the pixel division into the two sub-pixels, and the light transmittance may be decreased as a voltage drop is caused due to the capacitance combination.